This invention relates generally to imaging systems capable of operation in multiple modalities, and more particularly to an apparatus and method for compressing imaging data generated by a multi-modality imaging system.
Multi-modality imaging systems are capable of scanning using different modalities, such as, for example, Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Computed Tomography (CT). During operation of a PET imaging system for example, a patient is initially injected with a radiopharmaceutical that emits positrons as it decays. The emitted positrons travel a relatively short distance before they encounter an electron, at which point an annihilation occurs whereby the electron and positron are annihilated and converted into two gamma rays each having an energy of 511 keV. The two gamma rays are directed in nearly opposite directions and detected approximately simultaneously by two separate detector crystals, also commonly referred to as scintillators or scintillator crystals, in the PET scanner.
During a typical scan, the PET imaging system detects and records millions of gamma ray events, referred to herein as racy data. The rate data is typically stored as a list of events in a list file or histogrammed and stored as a histogram file. The list files may include fifteen million events per second resulting in the files being relatively large in size. The rack data is processed by a coincidence processor module to determine and record annihilation events. Typically the coincidence processor module analyzes the racy data to determine if any two of the individual gamma ray events occurred within a small pre-determined time window. If two gamma ray events are detected within a relatively short timeframe, referred to as a coincidence timing window, an event called a prompt is recorded along the line connecting the two crystals, sometimes referred to as a line-of-response (LOR).
The coincidence processor module may determine that of fifteen million gamma ray events detected, five million annihilation events have occurred. While the resulting list file of annihilation events is typically smaller than a file that includes only single gamma ray detected events, the list file of annihilation events is still typically several gigabytes in size. In one case, the list file of annihilation events is transmitted to a histogrammer. For each annihilation event, the histogrammer reads the current count value from a cell of the histogram, modifies the count value by incrementing or decrementing it, and writes the modified value back to the cell.
As imaging system technology advances the quantity of rack data increases. For example, the introduction of higher resolution detectors having more but smaller detector crystals, faster scintillators/detectors to improve sensitivity and count rate capability increased Axial FOVs, etc. As a result, of technological advances, the data word size of a coincidence event has increased and will continue to increase for PET scanners of the future. The previously stated PET Imaging scanner trends toward greater number of detector crystals, and increased Time-Of-Flight (TOF) timing precision and resolution, etc., coupled with the increased reliance on list files, retrospective list replay to produce gated, dynamic and/or static frames of data from a single PET scan are factors that will increase the reliance on list data and therefore increase the cost of streaming and storing list data. As a result, the increased quantity of raw data results in an increased processing demand on the histogrammer as will as an increased demand for additional storage space to store list files having a greater size.